netdev - RE: [PATCH] Force speed and duplex mode setting if link down

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D17256451@AcuExch.aculab.com>
Date:	Tue, 3 Jun 2014 09:13:17 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Sam Tannous' <stannous@...ulusnetworks.com>,
	Florian Fainelli <f.fainelli@...il.com>
CC:	netdev <netdev@...r.kernel.org>,
	Shrijeet Mukherjee <shm@...ulusnetworks.com>,
	Scott Feldman <sfeldma@...ulusnetworks.com>
Subject: RE: [PATCH] Force speed and duplex mode setting if link down

From: Of Sam Tannous
> 
> I probably didn't make this clear enough in the description.
> I'm not describing a use case.   I'm simply changing the
> speed on two interfaces that are directly connected.
> I don't have autoneg on at all.
> 
> 1. I start with a working link where the speed is
>    10G and duplex is full on both sides.
> 
> 2. I change the speed on one side to 1G with
> 
>       ethtool -s speed 1000
> 
>    and the command appears to work (at least there is no error message).
>    Once I issue this command, the link is no longer usable
>    since the speed changed on one side but not the other.  Autoneg
>    is disabled (by default).

You really want to leave autoneg enabled and just limit the advertised
speeds.

I suspect there are plenty of ways ethtool can be used to make
non-working interfaces, some of them might be needed be developers
or in unusual circumstances.
So I don't think ethtool should be policing this specific condition.

Maybe the driver should just use FDX when HDX is invalid for the
speed.

	David